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Monitoring lead acid batteries

Freescale Semiconductor has just introduced an intelligent battery sensor, and when I first heard about it I assumed that it was for lithium ion batteries. Not so. The MM912J637 measures the voltage, current and temperature of lead-acid batteries – the kind that will continue to be common in cars for who knows how many years.

Product marketing manager Antonio Leone says intelligent battery sensors have been niche products concentrated at the luxury end of the automotive market, but that is likely to change given increased electrical loads in cars these days and greater emphasis placed on fuel economy and CO2 reduction. Leone predicts that in Europe, especially, virtually all cars will come with an intelligent battery sensor.

A battery must be able to provide enough energy to crank the engine and be available as a power source to support new functions such as start-stop and intelligent alternator control. Leone says vehicle breakdowns caused by the electrical system can usually be traced back to the battery and can generally be avoided by knowing the precise state of the battery. Freescale’s device sends information on battery health to a body control module via LIN protocol. That information can be used to predict how long the battery ls likely to last, and whether or not some systems should be turned off temporarily.

The MM912J637 combines an S12 microcontroller and a SMARTMOS analog control IC, and needs just six external components, including a shunt resistor at the negative pole for current measurement and a series resistor at the positive pole for voltage measurement. A two-channel, 16-bit analog-to-digital converter provides simultaneous measurement of battery voltage and current and an independent 16-bit ADC adds temperature measurement. The package is small enough (7x7mm QFN) to fit on the battery’s negative pole. It is AEC-Q100 qualified for operation from -40°C to +125°C In “a very noisy environment.”

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John Day recently launched John Day’s Automotive Electronics News (johndayautomotivelectronics.com) to provide news and feature coverage of the automotive electronics industry. Earlier he wrote for Auto Electronics magazine, Auto E-lectronics, EE Times, and other business and engineering publications.
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